Graphene-Coated Hard Carbon Anode Material and Its Preparation Method

Abstract

The present invention discloses a graphene-coated hard carbon negative electrode material and a preparation method thereof, belonging to the technical field of hard carbon negative electrode materials. The preparation method of the graphene-coated hard carbon negative electrode material comprises subjecting coconut shells to pretreatment and three-step soaking, followed by biomass carbonization, and then mixing with a microbial suspension and adsorbing to obtain a microbial adsorption-type hard carbon material; then subjecting the microbial adsorption-type hard carbon material to surface treatment, chemical vapor deposition, impregnation, re-chemical vapor deposition and annealing treatment to obtain the graphene-coated hard carbon negative electrode material. The negative electrode material prepared by the present invention has a high initial discharge capacity, a high initial Coulomb efficiency and good cycling performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of hard carbon anode materials, and particularly relates to a graphene-coated hard carbon anode material and a preparation method thereof. Background Art

[0002] With the continuous development of modern technology, the overexploitation and utilization of traditional fossil fuels such as coal, oil, and natural gas inevitably cause energy crises and environmental pollution problems. In recent years, new green, inexpensive, renewable, and sustainable energy sources, including solar energy, biomass energy, geothermal energy, wind energy, and tidal energy, have received extensive attention from scientists. However, these new energy sources, which are highly dependent on weather, seasons, and locations, have characteristics such as intermittency and instability. To achieve continuous and stable power output, they need to be first converted into electrical energy and then stored in large-scale energy storage systems. Therefore, the development of efficient and convenient large-scale energy storage technologies plays a crucial role in the development and utilization of new energy sources.

[0003] As a key carrier of large-scale energy storage technology, the performance of batteries directly affects the energy storage effect. As an important component of batteries, anode materials play a decisive role in the overall performance of batteries. Finding high-performance anode materials has become a key link in improving the energy storage capacity of batteries and promoting the development of large-scale energy storage technologies. Among many anode materials, hard carbon has gradually attracted the attention of the industry due to its unique and excellent properties and application potential.

[0004] Hard carbon anode materials play a significant role in improving the energy storage performance of batteries, enhancing safety, and reducing costs. They are widely used in lithium-ion batteries and sodium-ion batteries and are thus widely applied in the field of new energy vehicles. Therefore, the present invention develops a graphene-coated hard carbon anode material and a preparation method thereof to solve the technical problems in the prior art, such as rapid capacity decay and poor conductivity of hard carbon anode materials during cycling due to improper treatment of hard carbon materials and uneven surface coating of hard carbon materials. Summary of the Invention

[0005] The purpose of the present invention is to provide a graphene-coated hard carbon anode material and a preparation method thereof to solve the technical problems in the prior art, such as rapid capacity decay and poor conductivity of hard carbon anode materials during cycling due to improper treatment of hard carbon materials and uneven surface coating of hard carbon materials.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A preparation method of a graphene-coated hard carbon anode material includes the following steps:

[0008] (1)Preparation of microbial adsorption type hard carbon material: After the coconut shell is pretreated and soaked in three steps, biomass carbonization is carried out, and then it is mixed with the microbial suspension and adsorbed to obtain the microbial adsorption type hard carbon material;

[0009] (2)Graphene coating: The microbial adsorption type hard carbon material is subjected to surface treatment, chemical vapor deposition, impregnation, secondary chemical vapor deposition and annealing treatment to obtain the graphene-coated hard carbon anode material.

[0010] Furthermore, the preparation method of the graphene-coated hard carbon anode material, step (1) includes the following process:

[0011] S1. Pretreatment of coconut shell: The coconut shell is washed with deionized water, then soaked in the surfactant solution and ultrasonically cleaned. After that, it is dried to obtain a clean coconut shell;

[0012] S2. Three-step soaking: The clean coconut shell is soaked in the HCl solution. After that, it is rinsed with deionized water until neutral, put into the NaOH solution for soaking, then rinsed with deionized water until neutral, and then soaked in the catalyst solution. After that, it is rinsed with deionized water and dried to obtain the chemically treated coconut shell;

[0013] S3. Carbonization: The chemically treated coconut shell is heated in nitrogen or argon and kept warm to obtain the hard carbon material;

[0014] S4. Microbial adsorption: The hard carbon material is dispersed in the microbial suspension, stirred and adsorbed. After that, it is dried and carbonized in an inert atmosphere to obtain the microbial adsorption type hard carbon material.

[0015] Further, in S1, it is washed with deionized water 2 - 3 times. The surfactant solution is an equal - volume mixture of sodium dodecylbenzenesulfonate solution and polyoxyethylene octylphenol ether solution. The mass concentration of the sodium dodecylbenzenesulfonate solution is 0.2 - 0.8 wt%, the mass concentration of the polyoxyethylene octylphenol ether solution is 0.3 - 1 wt%, the ultrasonic cleaning time is 1 - 2 hours, the drying temperature is 60 - 80 °C, and the drying time is 12 - 24 hours; in S2, the concentration of the HCl solution is 0.1 - 1 mol / L, the concentration of the NaOH solution is 0.1 - 1 mol / L, the soaking time is 2 - 4 hours. The catalyst solution is prepared by mixing ferric nitrate and nickel nitrate in a mass ratio of 1 - 2:1 - 2, the mass concentration of the catalyst solution is 0.8 - 3 wt%, the soaking time of the catalyst solution is 12 - 24 hours, the drying temperature is 60 - 80 °C, and the drying time is 12 - 24 hours; in S3, it is heated to 800 - 1200 °C, the heating rate is 2 - 4 °C / min, and the heat - preservation time is 1 - 3 hours; in S4, the microbial suspension is an equal - amount mixture of Rhizopus and Penicillium, the dry - weight concentration of the mycelium is 0.5 - 2 g / L, the adsorption time is 12 - 24 hours, the inert atmosphere is nitrogen or argon, the carbonization temperature is 800 - 1200 °C, the heating rate is 2 - 4 °C / min, and the heat - preservation time is 1 - 3 hours.

[0016] Further, the preparation method of the graphene - coated hard - carbon anode material, step (2) includes the following process:

[0017] M1. Surface treatment of hard carbon: Immerse the microbial - adsorption - type hard - carbon material in a mixed - acid solution, then wash it with deionized water until neutral, and dry it to obtain acid - leached hard - carbon material. Then place the acid - leached hard - carbon material in a plasma treatment device, and introduce oxygen and argon to obtain activated hard - carbon material;

[0018] M2. Primary chemical vapor deposition: Place the activated hard - carbon material in a quartz boat in a chemical vapor deposition reaction furnace, put the quartz boat in the heating area of the reaction furnace, close the reaction furnace, first introduce argon to purge the reaction furnace, heat up. When the temperature reaches the set value, keep the temperature constant, then introduce a carbon - source gas and hydrogen. After the reaction ends, close the carbon - source gas, continue to introduce hydrogen and argon, cool down. Wait until the reaction furnace cools to room temperature to obtain the primary graphene - coated hard - carbon material;

[0019] M3. Impregnation: Immerse the primary graphene - coated hard - carbon material completely in a polyvinylidene fluoride solution, take it out and dry it. Repeat the impregnation to obtain the impregnated - coated hard - carbon material;

[0020] M4. Secondary chemical vapor deposition: Repeat process M2 to obtain the secondary graphene - coated hard - carbon material;

[0021] M5. Annealing treatment: Put the graphene-coated hard carbon material into a high-temperature furnace and conduct annealing treatment under an argon or nitrogen protective atmosphere to obtain the graphene-coated hard carbon negative electrode material.

[0022] Furthermore, in M1, the mixed acid solution is prepared by mixing concentrated sulfuric acid and concentrated nitric acid according to a volume ratio of 3:1. The drying temperature is 60 - 80 °C, the drying time is 12 - 24 hours, the plasma treatment time is 5 - 10 minutes, the gas flow rate is 20 - 50 sccm, and the plasma power is 50 - 150 W; in M2, the flow rate of argon is controlled at 200 - 500 sccm, the purging time of the reaction furnace is 15 - 30 minutes, the heating rate is 5 - 10 °C / min, heat up to 900 - 1000 °C, after the temperature reaches the set value, keep the temperature for 10 - 15 minutes, the carbon source gas is methane or ethylene, the volume ratio of the carbon source gas to hydrogen is 1:10 - 20, the total gas flow rate is 100 - 300 sccm, the reaction time is 30 - 60 minutes, and the cooling rate is 5 - 10 °C / min; in M3, the polyvinylidene fluoride solution is prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone, the mass concentration of the polyvinylidene fluoride solution is 5 - 15 wt%, the immersion time is 10 - 30 minutes, the drying temperature is 80 - 100 °C, the drying time is 10 - 16 hours, and repeat the impregnation 2 - 3 times; in M5, the annealing temperature is 300 - 500 °C, and the annealing time is 1 - 3 hours.

[0023] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are:

[0024] 1. In the present invention, coconut shells are pretreated and soaked in three steps, followed by biomass carbonization, and then mixed with a microbial suspension for adsorption to obtain a microbial adsorption-type hard carbon material. Among them, the HCl solution can dissolve some basic impurities such as metal oxides and carbonates in the coconut shells, and at the same time can etch the surface of the coconut shells, increasing their specific surface area and surface roughness, which is beneficial to the adsorption and reaction of subsequent substances; the NaOH solution can react with lignin, hemicellulose, etc. in the coconut shells, making the structure of the coconut shells more porous, further increasing the specific surface area, and at the same time providing more active sites for the subsequent loading of catalysts; iron and nickel can catalyze the rearrangement and graphitization of carbon atoms during the carbonization process, contributing to the formation of a hard carbon material with a good crystalline structure and conductivity; in addition, microorganisms have the ability to adsorb and metabolize organic substances. Rhizopus and Penicillium can grow and adsorb on the surface and pores of the hard carbon material, and the mycelia can entangle on the hard carbon particles to form a network structure, increasing the specific surface area and porosity of the material. At the same time, microorganisms secrete some organic substances during growth, which can be further carbonized into carbonaceous materials during the carbonization process, improving the performance of the hard carbon material. Finally, the carbonaceous materials formed after microbial carbonization are intertwined with the original hard carbon material, increasing the structural stability and conductivity of the material, and at the same time providing more active sites and channels for the storage of sodium ions.

[0025] 2. Through the surface treatment of the microbial adsorption-type hard carbon material in the present invention, first, soaking in a mixed acid can remove impurities, oil stains, and some amorphous carbon on the surface of the hard carbon, and at the same time introduce oxygen-containing functional groups such as carboxyl and hydroxyl groups on the surface of the hard carbon, which not only increases the active sites on the surface of the hard carbon but also helps the subsequent uniform coating of graphene; secondly, in the plasma treatment, oxygen plasma can etch the surface of the hard carbon, increasing the surface roughness and surface energy, and argon plasma can play a role in cleaning and bombarding the surface of the hard carbon, further removing surface pollutants, and at the same time generating more defects and active centers on the surface, which is beneficial to the adsorption and decomposition of carbon source gases during the subsequent chemical vapor deposition process.

[0026] 3. In the present invention, through chemical vapor deposition, carbon atoms pyrolyzed from carbon source gases nucleate and grow into graphene layers on the surface of the hard carbon material. The graphene layers tightly coat the surface of the hard carbon particles, forming a continuous conductive network; in addition, through impregnation with a polyvinylidene fluoride solution, polyvinylidene fluoride can be filled between the graphene layer and the hard carbon material and in the pores of the graphene layer, playing a role in bonding and protection, enhancing the overall structural stability of the material, and at the same time helping to inhibit side reactions between the electrode material and the electrolyte; secondary vapor deposition can further improve the graphene coating layer, making the coating more uniform and dense, enhancing the conductivity and structural integrity of the electrode material, thereby improving the rate performance and cycle stability of the negative electrode material. Detailed implementation method

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0028] Example 1. The present example discloses a preparation method of a graphene-coated hard carbon negative electrode material, which includes the following steps:

[0029] (1) Preparation of a microbial adsorption type hard carbon material: After the coconut shell is pretreated and soaked in three steps, biomass carbonization is carried out, and then it is mixed with a microbial suspension and adsorbed to obtain a microbial adsorption type hard carbon material;

[0030] (2) Graphene coating: The microbial adsorption type hard carbon material is subjected to surface treatment, chemical vapor deposition, impregnation, re-chemical vapor deposition and annealing treatment to obtain a graphene-coated hard carbon negative electrode material.

[0031] Among them, the preparation method of the graphene-coated hard carbon negative electrode material, step (1) includes the following process:

[0032] S1. Pretreatment of coconut shell: The coconut shell is washed twice with deionized water, and then soaked in a surfactant solution. The surfactant solution is an equal-volume mixture of a sodium dodecyl benzene sulfonate solution and a polyoxyethylene octylphenol ether solution. The mass concentration of the sodium dodecyl benzene sulfonate solution is 0.2 wt%, and the mass concentration of the polyoxyethylene octylphenol ether solution is 0.3 wt%. Ultrasonic cleaning is carried out for 1 hour. After that, it is dried. The drying temperature is 60 °C, and the drying time is 12 hours to obtain a clean coconut shell;

[0033] S2. Three-step soaking: The clean coconut shell is soaked in a 0.1 mol / L HCl solution for 2 hours, rinsed with deionized water until neutral, put into a 0.1 mol / L NaOH solution and soaked for 2 hours, then rinsed with deionized water until neutral, and then soaked in a catalyst solution. The catalyst solution is prepared by mixing ferric nitrate and nickel nitrate according to a mass ratio of 1:2. The mass concentration of the catalyst solution is 0.8 wt%. The soaking time of the catalyst solution is 12 hours. It is rinsed with deionized water and dried. The drying temperature is 60 °C, and the drying time is 12 hours to obtain a chemically treated coconut shell;

[0034] S3. Carbonization: The chemically treated coconut shell is heated to 800 °C in nitrogen or argon, and the heating rate is 2 °C / min, and it is kept warm for 1 hour to obtain a hard carbon material;

[0035] S4. Microbial adsorption: Disperse the hard carbon material in the microbial suspension, which is an equal mixture of Rhizopus and Penicillium, with the dry weight concentration of the mycelium being 0.5 g / L. Stir for adsorption for 12 hours. After that, dry it and carbonize it under nitrogen or argon. The carbonization temperature is 800 °C, the heating rate is 2 °C / min, and keep it at this temperature for 1 hour to obtain the microbial adsorption type hard carbon material.

[0036] Among them, the preparation method of the graphene-coated hard carbon negative electrode material in step (2) includes the following process:

[0037] M1. Surface treatment of hard carbon: Immerse the microbial adsorption type hard carbon material in the mixed acid solution, which is prepared by mixing concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1. Then wash it with deionized water until neutral, and dry it. The drying temperature is 60 °C and the drying time is 12 hours to obtain the acid-leached hard carbon material. Then place the acid-leached hard carbon material in the plasma treatment equipment, introduce oxygen and argon, the plasma treatment time is 5 minutes, the gas flow rate is 20 sccm, and the plasma power is 50 W to obtain the activated hard carbon material;

[0038] M2. First chemical vapor deposition: Place the activated hard carbon material in the quartz boat of the chemical vapor deposition reaction furnace, put the quartz boat into the heating area of the reaction furnace, close the reaction furnace, first introduce argon, control the flow rate of argon at 200 sccm, purge the reaction furnace for 15 minutes, heat up, the heating rate is 5 °C / min, heat up to 900 °C. When the temperature reaches the set value, keep it at this temperature for 10 minutes, then introduce methane and hydrogen, the volume ratio of the carbon source gas to hydrogen is 1:10, the total gas flow rate is 100 sccm, the reaction time is 30 minutes, close the carbon source gas, continue to introduce hydrogen and argon, cool down, the cooling rate is 5 °C / min. Wait for the reaction furnace to cool to room temperature to obtain the first graphene-coated hard carbon material;

[0039] M3. Impregnation: Immerse the first graphene-coated hard carbon material completely in the polyvinylidene fluoride solution, which is prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone. The mass concentration of the polyvinylidene fluoride solution is 5 wt%, immerse it for 10 minutes, take it out and dry it. The drying temperature is 80 °C and the drying time is 10 hours. Repeat the impregnation 2 times to obtain the impregnated and coated hard carbon material;

[0040] M4. Second chemical vapor deposition: Repeat process M2 to obtain the second graphene-coated hard carbon material;

[0041] M5. Annealing treatment: Put the graphene-coated hard carbon material into a high-temperature furnace, and carry out annealing treatment under the protection of argon or nitrogen atmosphere. The annealing temperature is 300 °C and the annealing time is 1 hour to obtain the graphene-coated hard carbon anode material.

[0042] Example 2. The present example discloses a preparation method of a graphene-coated hard carbon anode material, including the following steps:

[0043] (1) Preparation of the microbial adsorption type hard carbon material: After the coconut shell is pretreated and soaked in three steps, carry out biomass carbonization, and then mix it with the microbial suspension and adsorb it to obtain the microbial adsorption type hard carbon material;

[0044] (2) Graphene coating: Carry out surface treatment, chemical vapor deposition, impregnation, re-chemical vapor deposition and annealing treatment on the microbial adsorption type hard carbon material to obtain the graphene-coated hard carbon anode material.

[0045] Among them, the preparation method of the graphene-coated hard carbon anode material, step (1) includes the following process:

[0046] S1. Pretreatment of coconut shell: Wash the coconut shell with deionized water 3 times, and then soak it in the surfactant solution. The surfactant solution is an equal-volume mixture of sodium dodecylbenzenesulfonate solution and polyoxyethylene octylphenol ether solution. The mass concentration of the sodium dodecylbenzenesulfonate solution is 0.5 wt%, and the mass concentration of the polyoxyethylene octylphenol ether solution is 0.7 wt%. Carry out ultrasonic cleaning for 1.5 hours. After that, dry it. The drying temperature is 70 °C and the drying time is 19 hours to obtain a clean coconut shell;

[0047] S2. Three-step soaking: Soak the clean coconut shell in 0.6 mol / L HCl solution for 3 hours, rinse it with deionized water until neutral, put it into 0.5 mol / L NaOH solution and soak it for 3 hours, then rinse it with deionized water until neutral, and then soak it in the catalyst solution. The catalyst solution is prepared by mixing ferric nitrate and nickel nitrate according to the mass ratio of 1:1.5. The mass concentration of the catalyst solution is 2 wt%. The soaking time of the catalyst solution is 20 hours. Rinse it with deionized water and dry it. The drying temperature is 70 °C and the drying time is 18 hours to obtain the chemically treated coconut shell;

[0048] S3. Carbonization: Heat the chemically treated coconut shell in nitrogen or argon to 1000 °C at a heating rate of 3 °C / min and keep it warm for 2 hours to obtain the hard carbon material;

[0049] S4. Microbial adsorption: Disperse the hard carbon material in the microbial suspension, which is an equal mixture of Rhizopus and Penicillium, with the dry weight concentration of the mycelium being 1.5 g / L. Stir for adsorption for 20 hours. After that, dry it and carbonize it under nitrogen or argon. The carbonization temperature is 1000 °C, the heating rate is 3 °C / min, and keep the temperature for 2 hours to obtain the microbial adsorption-type hard carbon material.

[0050] Among them, the preparation method of the graphene-coated hard carbon anode material, step (2) includes the following process:

[0051] M1. Surface treatment of hard carbon: Immerse the microbial adsorption-type hard carbon material in the mixed acid solution, which is prepared by mixing concentrated sulfuric acid and concentrated nitric acid according to a volume ratio of 3:1. Then wash it with deionized water until neutral, and dry it at a drying temperature of 70 °C for 18 hours to obtain the acid-leached hard carbon material. Then place the acid-leached hard carbon material in a plasma treatment device, introduce oxygen and argon, with the plasma treatment time being 8 minutes, the gas flow rate being 40 sccm, and the plasma power being 100 W to obtain the activated hard carbon material;

[0052] M2. Primary chemical vapor deposition: Place the activated hard carbon material in the quartz boat of the chemical vapor deposition reaction furnace, put the quartz boat into the heating area of the reaction furnace, close the reaction furnace, first introduce argon, control the argon flow rate at 400 sccm, purge the reaction furnace for 25 minutes, heat up at a heating rate of 7 °C / min to 950 °C. When the temperature reaches the set value, keep the temperature for 13 minutes, then introduce methane and hydrogen. The volume ratio of the carbon source gas to hydrogen is 1:15, the total gas flow rate is 200 sccm, the reaction time is 50 minutes, close the carbon source gas, continue to introduce hydrogen and argon, cool down at a cooling rate of 8 °C / min. Wait for the reaction furnace to cool to room temperature to obtain the primary graphene-coated hard carbon material;

[0053] M3. Impregnation: Immerse the primary graphene-coated hard carbon material completely in the polyvinylidene fluoride solution, which is prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone. The mass concentration of the polyvinylidene fluoride solution is 10 wt%, immerse for 20 minutes, take it out, dry it at a drying temperature of 90 °C for 14 hours, and repeat the impregnation 3 times to obtain the impregnation-coated hard carbon material;

[0054] M4. Secondary chemical vapor deposition: Repeat process M2 to obtain the secondary graphene-coated hard carbon material;

[0055] M5. Annealing treatment: Put the graphene-coated hard carbon material into a high-temperature furnace, and under the protection of argon or nitrogen atmosphere, conduct annealing treatment at a temperature of 400 °C for 2 hours to obtain the graphene-coated hard carbon negative electrode material.

[0056] Example 3. This example discloses a preparation method of a graphene-coated hard carbon negative electrode material, which includes the following steps:

[0057] (1) Preparation of the microorganism-adsorbed hard carbon material: After the coconut shell is pretreated and soaked in three steps, conduct biomass carbonization, and then mix it with the microorganism suspension and adsorb it to obtain the microorganism-adsorbed hard carbon material;

[0058] (2) Graphene coating: Subject the microorganism-adsorbed hard carbon material to surface treatment, chemical vapor deposition, impregnation, re-chemical vapor deposition, and annealing treatment to obtain the graphene-coated hard carbon negative electrode material.

[0059] Among them, for the preparation method of the graphene-coated hard carbon negative electrode material, step (1) includes the following process:

[0060] S1. Pretreatment of coconut shell: Wash the coconut shell with deionized water 3 times, and then soak it in the surfactant solution. The surfactant solution is an equal-volume mixture of sodium dodecylbenzenesulfonate solution and polyoxyethylene octylphenol ether solution. The mass concentration of the sodium dodecylbenzenesulfonate solution is 0.8 wt%, and the mass concentration of the polyoxyethylene octylphenol ether solution is 1 wt%. Conduct ultrasonic cleaning for 2 hours. After that, dry it at a temperature of 80 °C for 24 hours to obtain a clean coconut shell;

[0061] S2. Three-step soaking: Soak the clean coconut shell in 1 mol / L HCl solution for 4 hours, rinse it with deionized water until neutral, put it into 1 mol / L NaOH solution and soak it for 4 hours, then rinse it with deionized water until neutral, and then soak it in the catalyst solution. The catalyst solution is prepared by mixing ferric nitrate and nickel nitrate according to the mass ratio of 1:1, and the mass concentration of the catalyst solution is 3 wt%. The soaking time of the catalyst solution is 24 hours. Rinse it with deionized water and dry it at a temperature of 80 °C for 24 hours to obtain the chemically treated coconut shell;

[0062] S3. Carbonization: Heat the chemically treated coconut shell in nitrogen or argon to 1200 °C at a heating rate of 4 °C / min and keep it warm for 3 hours to obtain the hard carbon material;

[0063] S4. Microbial adsorption: Disperse the hard carbon material in the microbial suspension. The microbial suspension is an equal mixture of Rhizopus and Penicillium, and the dry weight concentration of the mycelium is 2 g / L. Stir for adsorption for 24 hours. After that, dry it and carbonize it under nitrogen or argon. The carbonization temperature is 1200 °C, the heating rate is 4 °C / min, and keep the temperature for 3 hours to obtain the microbial adsorption-type hard carbon material.

[0064] Among them, the preparation method of the graphene-coated hard carbon negative electrode material in step (2) includes the following process:

[0065] M1. Surface treatment of hard carbon: Immerse the microbial adsorption-type hard carbon material in the mixed acid solution. The mixed acid solution is prepared by mixing concentrated sulfuric acid and concentrated nitric acid according to a volume ratio of 3:1. Then wash it with deionized water until neutral, and dry it. The drying temperature is 80 °C and the drying time is 24 hours to obtain the acid-leached hard carbon material. Then place the acid-leached hard carbon material in a plasma treatment device, introduce oxygen and argon, the plasma treatment time is 10 minutes, the gas flow rate is 50 sccm, and the plasma power is 150 W to obtain the activated hard carbon material;

[0066] M2. Primary chemical vapor deposition: Place the activated hard carbon material in the quartz boat of the chemical vapor deposition reaction furnace, put the quartz boat into the heating area of the reaction furnace, close the reaction furnace, first introduce argon, and control the flow rate of argon at 500 sccm to purge the reaction furnace for 30 minutes. Then heat up, and the heating rate is 10 °C / min. Heat up to 1000 °C. When the temperature reaches the set value, keep the temperature for 15 minutes, and then introduce ethylene and hydrogen. The volume ratio of the carbon source gas to hydrogen is 1:20, the total gas flow rate is 300 sccm, and the reaction time is 60 minutes. Close the carbon source gas, continue to introduce hydrogen and argon, and cool down. The cooling rate is 10 °C / min. Wait for the reaction furnace to cool to room temperature to obtain the primary graphene-coated hard carbon material;

[0067] M3. Impregnation: Immerse the primary graphene-coated hard carbon material completely in the polyvinylidene fluoride solution. The polyvinylidene fluoride solution is prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone. The mass concentration of the polyvinylidene fluoride solution is 15 wt%. The immersion time is 30 minutes, then take it out and dry it. The drying temperature is 100 °C and the drying time is 16 hours. Repeat the impregnation 3 times to obtain the impregnated-coated hard carbon material;

[0068] M4. Secondary chemical vapor deposition: Repeat process M2 to obtain the secondary graphene-coated hard carbon material;

[0069] M5. Annealing treatment: Put the graphene-coated hard carbon material into a high-temperature furnace, and carry out annealing treatment under the protection atmosphere of argon or nitrogen. The annealing temperature is 500 °C, and the annealing time is 3 hours to obtain the graphene-coated hard carbon anode material.

[0070] Comparative Example 1:

[0071] Compared with Example 2, in the preparation process of the graphene-coated hard carbon anode material in Comparative Example 1, the three-step soaking treatment is not carried out, and other conditions remain unchanged.

[0072] Comparative Example 2:

[0073] Compared with Example 2, in the preparation process of the graphene-coated hard carbon anode material in Comparative Example 2, the microbial adsorption treatment is not carried out, and other conditions remain unchanged.

[0074] Comparative Example 3:

[0075] Compared with Example 2, in the preparation process of the graphene-coated hard carbon anode material in Comparative Example 3, the polyvinylidene fluoride solution impregnation treatment is not carried out, and other conditions remain unchanged.

[0076] Experimental Example:

[0077] Test the performance of the graphene-coated hard carbon anode materials prepared in Examples 1-3 and Comparative Examples 1-3. Mix the graphene-coated hard carbon anode materials, acetylene black, and polyvinylidene fluoride prepared in Examples 1-3 and Comparative Examples 1-3 according to a mass ratio of 8:1:1 in an N-methylpyrrolidone solvent, and stir evenly to make a slurry. Coat the slurry evenly on a copper foil current collector, then dry it in a vacuum drying oven at 100 °C for 24 hours, and then punch it into a circular electrode sheet with a diameter of 12 mm. In a glove box filled with argon, a sodium sheet is used as the counter electrode, a polypropylene microporous membrane is used as the separator, and the electrolyte is a solution of 1 M NaPF6 dissolved in diethylene glycol dimethyl ether to assemble a button-type half-cell. Conduct a simulated battery test in a Neware battery test cabinet at 5 V and 10 mA, with a charge-discharge voltage of 0.01-3.0 V and a charge-discharge rate of 0.1 C. The test results are shown in Table 1:

[0078] Table 1

[0079]

[0080] It can be seen from the test results in Table 1 that the graphene-coated hard carbon anode materials prepared in Examples 1-3 of the present invention have a high initial discharge capacity, a high initial Coulombic efficiency, and good cycling performance. It can be known from the comparison between Comparative Examples 1-3 and Examples 1-3 that the first discharge capacity, the first Coulombic efficiency, and the cycling performance of the graphene-coated hard carbon anode material can be effectively improved by the three-step soaking treatment, the microbial adsorption treatment, and the polyvinylidene fluoride solution impregnation treatment.

[0081] As described above, it is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes should be covered within the protection scope of the present invention.

[0082] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the present invention to only the specific embodiments. Obviously, many modifications and changes can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and utilize the present invention well. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. Preparation method of graphene-coated hard carbon negative electrode material, characterized in that, It includes the following steps: (1) Preparation of microbial adsorption type hard carbon material: The coconut shell is pretreated and soaked in three steps, followed by biomass carbonization, and then mixed with a microbial suspension for adsorption to obtain the microbial adsorption type hard carbon material; (2) Graphene coating: The microbial adsorption type hard carbon material is subjected to surface treatment, chemical vapor deposition, impregnation, secondary chemical vapor deposition, and annealing treatment to obtain the graphene-coated hard carbon anode material; The step (1) includes the following process: S1. Pretreatment of coconut shell: The coconut shell is washed with deionized water, then soaked in a surfactant solution and ultrasonically cleaned. After that, it is dried to obtain a clean coconut shell; S2. Three-step soaking: The clean coconut shell is soaked in an HCl solution. After that, it is rinsed with deionized water until neutral, then soaked in an NaOH solution, and then rinsed with deionized water until neutral. Then it is soaked in a catalyst solution. After that, it is rinsed with deionized water and dried to obtain the chemically treated coconut shell; S3. Carbonization: The chemically treated coconut shell is heated in nitrogen or argon and kept at a constant temperature to obtain the hard carbon material; S4. Microbial adsorption: The hard carbon material is dispersed in a microbial suspension, stirred for adsorption. After that, it is dried and carbonized under an inert atmosphere to obtain the microbial adsorption type hard carbon material; In S4, the microbial suspension is an equal mixture of Rhizopus and Penicillium, and the dry weight concentration of the mycelium is 0.5 - 2 g / L; The step (2) includes the following process: M1. Surface treatment of hard carbon: The microbial adsorption type hard carbon material is soaked in a mixed acid solution, then rinsed with deionized water until neutral and dried to obtain the acid-leached hard carbon material. Then the acid-leached hard carbon material is placed in a plasma treatment device, and oxygen and argon are introduced to obtain the activated hard carbon material; M2. Primary chemical vapor deposition: The activated hard carbon material is placed in a quartz boat in a chemical vapor deposition reaction furnace. The quartz boat is placed in the heating area of the reaction furnace. The reaction furnace is closed. First, argon is introduced to purge the reaction furnace, and the temperature is raised. When the temperature reaches the set value, it is kept at a constant temperature. Then a carbon source gas and hydrogen are introduced. After the reaction ends, the carbon source gas is closed, and hydrogen and argon are continuously introduced. The temperature is lowered. When the reaction furnace cools to room temperature, the primary graphene-coated hard carbon material is obtained; M3. Impregnation: The primary graphene-coated hard carbon material is completely immersed in a polyvinylidene fluoride solution, taken out and dried. The impregnation is repeated to obtain the impregnation-coated hard carbon material; M4. Secondary chemical vapor deposition: Repeat the process M2 to obtain the secondary graphene-coated hard carbon material; M5. Annealing treatment: The graphene-coated hard carbon material is placed in a high-temperature furnace and annealed under an argon or nitrogen protective atmosphere to obtain the graphene-coated hard carbon anode material.

2. The preparation method of the graphene-coated hard carbon negative electrode material according to claim 1, wherein, In S1, it is washed with deionized water 2 - 3 times. The surfactant solution is an equal-volume mixture of a sodium dodecylbenzenesulfonate solution and a polyoxyethylene octylphenol ether solution. The mass concentration of the sodium dodecylbenzenesulfonate solution is 0.2 - 0.8 wt%, and the mass concentration of the polyoxyethylene octylphenol ether solution is 0.3 - 1 wt%.

3. The preparation method of the graphene-coated hard carbon negative electrode material according to claim 1, wherein, In S2, the concentration of the HCl solution is 0.1 - 1 mol / L, the concentration of the NaOH solution is 0.1 - 1 mol / L, the soaking time is 2 - 4 hours. The catalyst solution is prepared by mixing ferric nitrate and nickel nitrate in a mass ratio of 1 - 2:1 - 2, the mass concentration of the catalyst solution is 0.8 - 3 wt%, and the soaking time of the catalyst solution is 12 - 24 hours; in S3, it is heated to 800 - 1200 °C, the heating rate is 2 - 4 °C / min, and the heat preservation time is 1 - 3 hours.

4. The preparation method of the graphene-coated hard carbon negative electrode material according to claim 1, wherein, In S4, the microbial suspension is an equal - amount mixture of Rhizopus and Penicillium, the dry - weight concentration of the mycelium is 0.5 - 2 g / L, the adsorption time is 12 - 24 hours, the inert atmosphere is nitrogen or argon, the carbonization temperature is 800 - 1200 °C, the heating rate is 2 - 4 °C / min, and the heat preservation time is 1 - 3 hours.

5. The preparation method of the graphene-coated hard carbon negative electrode material according to claim 1, wherein, In M1, the mixed - acid solution is prepared by mixing concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1, the drying temperature is 60 - 80 °C, the drying time is 12 - 24 hours, the plasma - treatment time is 5 - 10 minutes, the gas flow rate is 20 - 50 sccm, and the plasma power is 50 - 150 W.

6. The preparation method of the graphene-coated hard carbon negative electrode material according to claim 1, characterized in that In M2, the flow rate of argon is controlled at 200 - 500 sccm, the time for purging the reaction furnace is 15 - 30 minutes, the heating rate is 5 - 10 °C / min, heated to 900 - 1000 °C. When the temperature reaches the set value, it is heat - preserved for 10 - 15 minutes. The carbon - source gas is methane or ethylene, the volume ratio of the carbon - source gas to hydrogen is 1:10 - 20, the total gas flow rate is 100 - 300 sccm, the reaction time is 30 - 60 minutes, and the cooling rate is 5 - 10 °C / min.

7. The preparation method of the graphene-coated hard carbon negative electrode material according to claim 1, wherein, In M3, the polyvinylidene fluoride solution is prepared by dissolving polyvinylidene fluoride in N - methylpyrrolidone, the mass concentration of the polyvinylidene fluoride solution is 5 - 15 wt%, the immersion time is 10 - 30 minutes, and it is repeatedly immersed 2 - 3 times; in M5, the annealing temperature is 300 - 500 °C, and the annealing time is 1 - 3 hours.

8. Graphene-coated hard carbon negative electrode material, characterized in that, It is prepared by using the preparation method of the graphite - coated hard - carbon negative - electrode material described in any one of claims 1 - 7.

Images